The present invention relates to a method for determining the interior shape of spherical tanks for the computation of the liquid volume of the tanks. The invention also relates to a combination of apparatus elements used in connection with the carrying out of the method.
It is known to use tanks of spherical shape for transporting liquified natural gas by ship. The determination of the volume of the liquified contents of such tanks requires a measuring method which provides adequate information about the interior shape of the spherical tank, so that from this information it is possible accurately to determine how much the tank at specified points deviates from the spherical form and accurately to compute the quantity of liquid in the tanks.
When copying with large spherical tanks the determination of the interior shape and the volume thereof involves a series of problems because the inner surface of the sphere differs from the theoretical spherical shape. This abberration is due to, inter alia, inaccuracies resulting from the manufacturing process causing bulges on the surface of the sphere, thick joints, possible "insides" of the sphere etc. Besides, the ambient temperature will influence the form and the volume of the spherical tanks, and the spherical tank will also be subjected to deformation due to the load of the contents of the tank.
A further problem which is present in the computation of the liquid volume of spherical tanks on ships is that the tank, apart from deviating from the theoretical spherical shape, turns with the trim of the ship, so that the liquid volume around the inner periphery of the sphere at different trim will cover different spherical areas, the shape of which may deviate from that of the spherical areas covered by the liquid contents of the tanks at zero trim. Besides, the measured length from the bearing plate to the float of the dipping means will be larger for a given volume at a trim value than when the ship is horizontal. Therefore, even if corrections are made both for the displacement to which the dipping means is subjected by trim and the eccentric location of the same along a vertical line, a given volume of liquid will indicate incorrect dipping values for the different trim values because the quantity of liquid is confined by different areas of the spherical tank where the latter may bulge to a larger or lesser degree.